National Overview

NCDC transitioned to the nClimDiv dataset on Thursday, March 13, 2014. This was coincident with the release of the February 2014 monthly monitoring report. For details on this transition, please visit our public FTP site and our U.S. Climate Divisional Database site.

During December the eastern third of the nation, as well as the Northern Plains, experienced above-average temperatures. The warmest temperature anomalies (the temperature compared to the 20th century average) occurred across the Northern Plains, the Great Lakes and into the Northeast. A total of 15 states had December temperatures ranking among their ten warmest.

Cooler-than-average temperatures were present for the Pacific Northwest, the West, and Southwest climate regions. New Mexico tied its fourth coolest December on record, with monthly temperatures 5.3 degrees F below average.

Much of the central part of the country was wetter than average, as a persistent storm track brought above-average precipitation to 18 states. Kansas (5th wettest), Arkansas (8th), and Ohio (8th) had December precipitation totals among their ten wettest.

The state of Alaska was warm and wet during December 2011. According to data back to 1918, this was the third warmest December on record with a temperature anomaly of 8.7 degrees F above the 1971-2000 average. The warmest December on record for Alaska occurred in 1985, when the monthly temperature anomaly was 11.0 degrees F above average. It was also the fifth wettest December on record for Alaska, with a precipitation anomaly of 39.4 percent above the 30-year average.

According to the U.S. Drought Monitor, as of January 3rd 2012, about 3.3 percent of the contiguous U.S. was experiencing the worst category of drought, called D4 or exceptional drought, a decrease from the nearly six percent at the beginning of the month. Drought conditions lessened across the Southwest and Southern Plains, where above average precipitation was observed during December. Moderate Drought conditions developed across parts of California, Nevada, and Oregon, where below-average precipitation was observed for several months.

During the last six months of 2011 (July-December), the average U.S. temperature was 59.1 degrees F or 1.9 degrees F above average, ranking as the fourth warmest such six-month period on record. The contiguous U.S. was drier than average for the period, with some regional variability.

The July-December period was wetter than average for much of the Ohio Valley and into the Northeast, consistent with long-term precipitation trends across these regions. Twelve states had six-month precipitation totals ranking among their ten wettest. Pennsylvania and New Jersey were record wet with precipitation anomalies of 16.85 inches and 11.34 inches, respectively.

During 2011, the contiguous U.S. was warmer than average, with a temperature anomaly of 1.0 degree F, ranking in the top third of the historical distribution. The U.S. as a whole had an average precipitation 0.36 inch below the 20th century average, ranking near the median of the historical distribution.

The South and Northeast had annual temperatures which were much-above average, having their 8th warmest and 9th warmest years on record, respectively. Sixteen states had annual temperatures among their ten warmest. Delaware was record warm for the period, with an annual temperature of 58.2 degrees F, or 3.5 degrees F above average. Texas had its second warmest year on record, with an annual temperature anomaly of 2.2 degrees F, just shy of the annual record of 2.5 degrees set in 1921.

Drier-than-average conditions were present across the southern tier of the country. Georgia (5th driest) New Mexico (6th), Louisiana (7th), and South Carolina (8th) had annual precipitation totals among their ten driest. Texas was record dry for the year, with 14.89 inches of precipitation — 13.03 inches below the 20th century average. The year 2011 surpassed 1917 as the driest year on record for Texas, when 14.99 inches of precipitation was observed across the state.

An in-depth report on the climate and weather of 2011 will be forthcoming in the NCDC’s 2011 Annual Report, scheduled to be released on January 19th.

Alaska Temperature and Precipitation:

Alaska
had its 3rd warmest December on record, with a temperature 8.7°F (4.8°C) above the 1971–2000 average.

Alaska
had its 35th warmest October-December on record, with a temperature 1.3°F (0.7°C) above the 1971–2000 average.

Alaska
had its 38th warmest January-December period on record, with a temperature 0.4°F (0.2°C) above the 1971–2000 average.

Alaska
had its 5th wettest December since records began in 1918, with an anomaly that was 39.4 percent below the 1971–2000 average.

Alaska
had its 24th wettest October-December on record, with an anomaly that was 7.8 percent above the 1971–2000 average.

Alaska
had its 36th wettest January-December period on record, with an anomaly that was 5.4 percent above the 1971–2000 average.

For additional details about recent temperatures and precipitation across the U.S., see the Regional Highlights section below. For information on local temperature and precipitation records during the month, please visit NCDC's Records page. For details and graphics on weather events across the U.S. and the globe please visit NCDC's Global Hazards page.

December’s average temperature of 34.1 degrees F (1.2 degrees C) was 5.3 degrees F (2.9 degrees C) above normal and 9.0 degrees F (5.0 degrees C) warmer than December 2010. It was the warmest December since 2006 (2006 was the warmest December since 1895) and the sixth warmest December since recordkeeping began in 1895. In addition, it was the ninth consecutive month to average warmer than normal. Each of the states in the Northeast was warmer than normal, with departures that ranged from +4.5 degrees F (+2.5 degrees C) in Maryland and West Virginia to +6.3 degrees F (+3.5 degrees C) in Delaware. This month was the 2nd warmest December since 1895 in Delaware and Rhode Island, and the 3rd warmest in Connecticut, Massachusetts, and New Jersey. Temperatures in the remaining states ranked within the 5th to 15th warmest on record. With nine of the twelve months averaging above normal, the yearly average was also warmer than normal. The Northeast’s average annual temperature in 2011 was 49.0 degrees F (9.4 degrees C), which was 1.5 degrees F (0.8 degrees C) above normal and only one degree F (0.6 degrees C) less than the warmest year, 1998. This year was the Northeast’s 9th warmest since 1895. Each of the states in the region ranked within the top 12 warmest since 1895, with Delaware posting its warmest year on record. Departures among the states ranged from +1.2 degrees F (+0.7 degrees C) in Massachusetts to +2.5 degrees F (+1.4 degrees C) in Delaware.

December’s precipitation total of 3.64 inches (92.5 mm) was 104 percent of normal. While this month’s total was 0.17 inches (4.3 mm) above normal, it was 0.32 inches (8.1 mm) less than December 2010. This was the fifth consecutive wetter-than-normal December. None of the state’s precipitation totals departed far from normal Delaware, at 94 percent of normal, was the driest state; and Pennsylvania was the wettest, with 113 percent of its normal precipitation amount. The Northeast’s annual precipitation total of 55.14 inches (1400.6 mm) sent 2011 into the record book as the wettest year since 1895, surpassing 1996’s total of 54.60 inches (1386.8 mm). This year’s total was 10.97 inches (278.6 mm) wetter than normal. Four of the states hit hard by the tropical storms in August and September - Connecticut, New Jersey, New York, and Pennsylvania - also had their wettest year on record. Vermont saw its 2nd wettest year since 1895 and Massachusetts, its 3rd. The region averaged 124 percent of normal in 2011; state departures ranged from 109 percent in Delaware to 136 percent in New Jersey and Pennsylvania.

Unlike December 2010, when most of the Northeast saw above normal snowfall, this December was noteworthy for its lack of snow. In a normal December, the northern half of the region gets at least a foot (30.5 cm) of snow. This month, a few areas in northern Maine and Vermont had totals of up to a foot (30.5 cm) and only a small section of New York east of Lake Ontario saw over 24 inches (61.0 cm). Normally snowy Syracuse, NY didn’t see its first inch (2.54 cm) of snow until the 15th; Buffalo, NY’s seasonal total of 3.8 inches (9.65 cm) was 32.9 inches (83.57 cm) below normal. Both Buffalo and Syracuse had their 3rd least snowy October through December on record. The lack of major snow events this month had its blessings and its curses. Local and state snow removal costs were minimal, holiday shoppers could get to the malls with ease, and weather related travel delays were few and far between, especially during the holiday season. On the flip side, the winter tourism industry was suffering financially due to the lack of snow and cold temperatures.

December temperatures averaged 3 to 10 degrees F (2 to 6 degrees C) above normal across the Midwest with the largest departures in Minnesota. Temperatures were below normal early in December but well above normal for the last two-thirds of the month. Daily temperature records were almost exclusively record highs with 629 record highs and just 7 record lows. Annual temperatures were very close to normal for the entire region with all locations within 2 degrees F (1 degree C) of normal for the year.

December precipitation ranged from less than 0.25 inches (6.4 mm) in parts of Minnesota to more than 7 inches (177.8 mm) in southeast Missouri and southwest Kentucky. The low totals in Minnesota were less than 25% of normal in some locations. In northwest Missouri and southern Iowa, precipitation totals of 2.5 to 3 inches (63.5 to 76.2 mm) more than doubled normal. Annual precipitation ranged from less than 20 inches (508.0 mm) in northwest Minnesota, 75% of normal, to more than 70 inches (1778.0 mm) along the Ohio River Valley, 150% of normal. Indiana, Ohio, and Kentucky recorded their wettest year statewide in 2011 (1895-2011 period). Ten cities along the Ohio River and northward to Lake Erie set new records for annual precipitation. Cincinnati, Ohio, with 140 years of data, broke its old record of 57.58 inches (1462.5 mm) in 1990 by 15.70 inches (398.8 mm) with 73.28 inches (1861.3 mm) in 2011. Cleveland, Ohio, with 141 years of data, broke its previous record of 53.83 inches (1367.3 mm) in 1990 by 11.49 inches (291.8 mm) with 65.32 inches (1659.1 mm) in 2011. Many other stations in this area set annual precipitation records including several in Ohio that topped the old state record of 70.82 inches (1798.8 mm) including Cheviot, Ohio with an unofficial total of 76.24 inches (1936.4 mm).

Snow totals were unusually low across the Midwest in December. Monthly totals were just 0 to 2 inches in the southern half of the region and topped 12 inches only in northern Wisconsin and Upper Michigan. Those totals were below normal by 0 to 5 inches (0 to 127 cm) for most of the region and 5 to 10 inches (127 to 154 cm) for the northern third of the region and as much as 24 inches (610 cm) below normal for parts of northern Michigan along the Great Lakes. By the end of the month, snow was on the ground only in parts of Minnesota, Wisconsin, and Michigan. . Winter sports in the upper Midwest have been hampered by the lack of snow and warmth which has melted much of what did fall. An upside has been lower heating costs for residential customers and reduced snow removal cost for municipal budgets.

Despite the lack of snow overall, hundreds of accidents have been reported in December, some in snow events with just an inch or two (3 to 5 cm) of snow. Freezing rain spread across a wide swath of the Midwest on the 29th and 30th. Dozens of accidents were reported in each of numerous cities from southern Wisconsin to northern Ohio.

Heavy rains in Indianapolis, Indiana led to one death on the 21st. The fatality was a drowning in a ditch.

Monthly average temperatures for December were above normal across the Southeast region. The greatest departures were found across the northern half of the region, where monthly temperatures were between 5 and 6 degrees F (2.8 to 3.3. degrees C) above average, while temperatures across the southern half of the region were between 2 and 4 degrees F (1.1 to 2.2 degrees C) above normal. Monthly temperatures were slightly above normal across much of Puerto Rico and the U.S. Virgin Islands. December ranked among the top 10 warmest on record at several locations across the region, including Tampa, FL, Miami, FL, Columbus, GA, Raleigh-Durham, NC, Greenville, SC, and Richmond, VA. Overnight temperatures were especially warm during the month, as over 200 daily high minimum temperature records were tied or broken across the region. Many of these occurred in the days before Christmas, which was in stark contrast to the cold and snow that struck the region one year earlier.

For the third straight month, precipitation was highly variable across the Southeast region. Monthly totals were between 100 and 150 percent of normal across western portions of Virginia and North Carolina and across sections of Alabama and North Georgia. Some locations across the Southern Appalachians recorded between 150 and 300 percent of normal precipitation for the month. Heavy precipitation connected with a slow-moving frontal system produced 4.28 inches (108.7 mm) at Meadows of Dan, VA on the 7th of the month, setting a monthly record 24-hr rainfall total. In contrast, the driest locations across the Southeast were found across the eastern Carolinas and the Florida Peninsula, where monthly precipitation totals were less than 25 percent of normal. Charleston, SC recorded only 0.65 inches (16.5 mm) of precipitation, making it the second driest December in a record extending back to 1938. Elsewhere across the region, monthly precipitation was between 25 and 75 percent of normal, including the western slopes and interior mountains of Puerto Rico. In contrast to last December, there was very little snowfall reported across the Southeast region. Trace amounts of snow were reported in northern Alabama and in the mountains of North Carolina and Virginia from the 7th to the 8th of the month in the wake of a frontal passage. Additionally, up to 2 inches (50.8 mm) of snow was reported across the higher elevations of North Carolina and Virginia as a low pressure system tracked across the eastern U.S. from the 27th to the 29th of the month.

The calendar year 2011 was exceptionally warm across a large portion of the Southeast region. Miami, FL and Cape Hatteras, NC recorded their warmest year ever in records extending back to 1895 and 1893, respectively. It was the second warmest year ever in Richmond, VA and Tampa, FL, while Columbia, SC and Norfolk, VA tied their second warmest year ever. Several other locations recorded one of their top 10 warmest years ever in 2011, including Atlanta, GA, Washington, D.C., Raleigh-Durham, NC, Charleston, SC, and Roanoke, VA. In terms of precipitation, it was the second driest year on record in Gainesville, FL and Tallahassee, FL, with preliminary annual totals of 33.28 inches (845.3 mm) and 34.81 inches (884.2 mm), respectively. One year after recording its wettest year ever, San Juan, PR recorded its second wettest year on record in 2011 with 88.16 inches (2239.3 mm).

There were 79 reports of severe weather across the Southeast in December, including an outbreak of 13 tornadoes on the 21st and 22nd of the month. Six of these tornadoes occurred in Georgia, including an EF-3 that tracked for several miles across Floyd, Gordon, and Bartow Counties northwest of Atlanta. One home was completely destroyed and several others were damaged. An EF-2 tornado was also reported in nearby Rome, GA, resulting in three injuries and significant damage to more than 20 homes. The remaining seven tornadoes were reported across central Alabama, including an EF-1 that tracked across Elmore, Coosa, and Tallapoosa Counties. This tornado destroyed a metal building and uprooted a 100 year old oak tree in Kellyton, AL. In total, between 400 and 500 trees were uprooted along its 25 mile (40.2 km) path.

Drought conditions continued to persist across more than half of the Southeast region in December. The lack of rainfall across the Florida Peninsula contributed to the re-emergence of D0 (abnormally dry) conditions by the end of the month, forcing several communities to consider implementing additional water restrictions. The warm, dry weather facilitated the planting and growth of winter grains and vegetables, as well as the transport of harvested crops across the region. A rare Code Orange air quality dsignation (considered unhealthy for sensitive groups according to the EPA) occurred across parts of central Georgia beginning on New Year’s Eve. Preliminary analyses by the Georgia State Climate Office suggest that the event was caused at least partly by local fireworks displays under ideal meteorological conditions (i.e. a strong nocturnal inversion, shallow boundary layer, and calm winds).

December was an interesting month across the High Plains Region. The northern areas of the region were generally drier and warmer than normal and many areas did not experience a White Christmas. The lack of snow cover, not just in the High Plains Region, but also in areas north into Canada, contributed to the unseasonably warm temperatures. The Dakotas had the largest temperature departures in the region, ranging from near normal in the southwest corner of South Dakota to over 12 degrees F (6.7 degrees C) above normal in northern and northeastern North Dakota. These warm temperatures caused many locations in the Dakotas to be ranked in the top 15 warmest Decembers on record. For instance, Fargo, North Dakota had an average temperature of 25.3 degrees F (-3.7 degrees C), which was 12.8 degrees F (7.1 degrees C) above normal. Fargo’s average temperature for December was ranked as the 3rd warmest and was just shy of the record which occurred in 1959 with 25.9 degrees F (-3.4 degrees C) (period of record 1881-2011). Meanwhile, the western areas of the Region were generally colder than normal with temperature departures at least 6 degrees F (3.3 degrees C) below normal occurring in areas of southern Colorado and Wyoming. A few locations in southern Colorado ranked in the top 10 coolest Decembers on record. Lamar, Colorado had its 3rd coolest December on record with an average temperature of 21.8 degrees F (-5.7 degrees C). While this was 8.5 degrees F (4.7 degrees C) below normal, it was not cool enough to beat the old December record of 18.1 degrees F (-7.7 degrees C) which occurred in 1924 (period of record 1893-2011).

Precipitation was varied across the Region this month. The Dakotas were generally drier than normal, with a few isolated pockets receiving above normal precipitation. The ongoing dry conditions in eastern North Dakota led to the expansion of moderate drought conditions (D1). Meanwhile, heavy rain and snow across Kansas and southern Colorado helped alleviate and in some cases eliminate drought conditions. Precipitation was a welcome sight in drought-stricken areas of Colorado, Kansas, and southeast Nebraska. This month, the heaviest precipitation in the Region fell in a large swath that stretched from southeastern Colorado, through Kansas, and into southeastern Nebraska. The majority of the locations in this swath received 200-400 percent of normal precipitation, while clusters of locations in Kansas and Colorado received 400-800 percent of normal precipitation. Numerous locations ranked in the top 10 wettest Decembers on record and many had precipitation totals which were ranked second and third wettest. Wichita, Kansas had its 5th wettest December with 3.69 inches (94 mm) of liquid equivalent precipitation (period of record 1888-2011). The wettest December occurred in 1984 with 4.71 inches (120 mm) of liquid equivalent precipitation. Snowfall totals for the month of December also ranked in the top 10 across some areas of Kansas and Colorado. At least one location had its snowiest December on record - Pueblo, Colorado. Pueblo received 18.6 inches (47 cm) of snow this month, which beat out the old record of 18.2 inches (46 cm) set all the way back 1913 (period of record 1888-2011). December 19-20 was an active period as a major winter storm brought heavy rain, sleet, and snow to Colorado and Kansas. Heavy snow was accompanied by strong winds which created blizzard conditions in some locations. According to the National Weather Service in Dodge City, Kansas, snow drifts of 8 to 10 feet (243 to 305 cm) were also reported. Some of the heaviest snow fell in southeastern Colorado where some interesting records were set. Lamar, Colorado received 19.0 inches (48 cm) of snow on December 20th and set an impressive new daily snowfall total for that day. This snow total well exceeded the old record of 10.0 inches (25 cm) set in 2006 (period of record 1893-2011). The snowfall on the 20th also set a new record as the highest one-day snowfall total for any day in December. Interestingly, the 19.0 inches (48 cm) of snow on the 20th was also the second highest one-day snowfall total for any day of the year in Lamar. The record one-day snowfall occurred October 26, 1997 when 22.0 inches (56 cm) fell in Lamar. 2011 ended on a windy note as damaging winds affected Wyoming, Colorado, Kansas, and Nebraska. Wind gusts over 60 mph (97 km/hr) were widespread and there were even reports of winds over 100 mph (161 km/hr) in some of the mountainous areas of Colorado. The high winds led to dangerous driving conditions on the open highways and some structural damage. According to the National Weather Service in Cheyenne, Wyoming, vehicles, buildings, and signs were damaged as a result of the high winds.

The U.S. Drought Monitor had many changes this month. Storm systems bringing beneficial rain and snow to Colorado and Kansas have led to the erasure of all exceptional drought conditions (D4) in those states. In addition, extreme drought conditions (D3) were also erased in Colorado. Other major changes occurred in Kansas as well, as all drought conditions were eliminated in the north and the drought conditions that remain have contracted to the south somewhat. Southeastern Nebraska also benefitted from the December precipitation as drought conditions were erased for a large portion of that area. Meanwhile, due to an ongoing lack of precipitation in North Dakota, moderate drought conditions (D1) have expanded northward in the eastern side of the state. Drought conditions in South Dakota remained largely the same since last month, with the exception of a slight expansion of abnormally dry (D0) and D1 conditions in the southeast. According to the U.S. Seasonal Drought Outlook drought conditions in portions of the Dakotas, Nebraska, Colorado, and western Kansas were expected to persist, while drought conditions in eastern Kansas were expected to improve.

The month of December, was generally cooler than normal in Texas, northern Louisiana and in the Oklahoma panhandle. Though it was cooler than normal, temperatures averaged only 0-4 degrees F (0 to 2.22 degrees C) lower than expected. Elsewhere, conditions were generally slightly warmer than normal, with temperatures averaging between 0 to 6 degrees F (0 to 3.33 degrees C) above expected values. The highest departures were observed in Tennessee, where most stations averaged between 4 and 6 degrees F (2.22 to 3.33 degrees C) above normal. The state of Tennessee had an average temperature of 42.20 degrees F (5.67 degrees C) for the month, and it was the thirty-second warmest December on record (1895-2011). On the flip side, Texas averaged only 45.40 degrees F (7.44 degrees C), which was the twenty-sixth coolest December on record (1895-2011). Other state average temperatures include: Arkansas at 42.60 degrees F (5.89 degrees C) , Louisiana at 50.80 degrees F (10.44 degrees C), Mississippi at 47.20 degrees F (8.44 degrees C), and Oklahoma at 38.90 degrees F (3.83 degrees C). The state rankings for these states all fell well within the two middle quartiles.

With a few exceptions, December was generally a wet month for the Southern Region. Dry areas included southern Louisiana and southern Mississippi, eastern Oklahoma, and parts of western Texas. These areas generally averaged approximately five to twenty five percent of normal precipitation for the month. The wettest part of the region included Arkansas, parts of southern and northern Texas, and western Oklahoma. Stations in these regions averaged between 150 and 400 percent of normal precipitation. Texas averaged 2.88 inches (73.15 mm) of precipitation, making it the nineteenth wettest December on record (1895-2011). It is also the second month in the year where precipitation for the state exceeded two inches (50.80 mm). Oklahoma experienced its twenty-eighth wettest December on record (1895-2011) with a state average precipitation total of 2.35 inches (59.69 mm). For Arkansas, it was the eighth wettest December on record (1895-2011) with a state average precipitation total of 7.47 inches (189.74 mm). Stations in the northeastern and southern areas of the state averaged between 150 and 250 percent of normal. Louisiana averaged 5.49 inches (139.45 mm) of precipitation, while Mississippi, averaged 5.99 inches (152.15 mm) of precipitation. Tennessee averaged 5.91 inches (150.11 mm) of precipitation for the month. State rankings for Louisiana, Mississippi, and Tennessee were all in the middle two quartiles.

Drought conditions in the Southern Region improved slightly from the previous month. Above normal precipitation over much of the region has led to improvements over western Louisiana, northwestern Texas, and northern Texas. As of January 3, 2012, only 17.24 percent of the Southern Region is experiencing exceptional drought, which is approximately a twelve percent improvement over the end of November. The amount of extreme drought was also reduced by approximately eleven percent. Drier than normal conditions in southeastern Louisiana, however; has led to the introduction of extreme drought over much of the Florida Parishes.

A snowstorm over December 19 to December 21, dumped up to 15 inches (381 mm) of snow across the great plains affecting areas from new Mexico to Kansas. In Oklahoma, and northern Texas, snow drifts were so high that many roads and interstates had to be temporarily closed.

On December 20, 2011, a tornado in Calcasieu Parish, Louisiana caused the roof of a patient care facility to be blown off.

Dry and generally cool conditions were observed in much of the West this month as high pressure dominated the region. Several systems made their way across the Southwest, bringing above normal precipitation to the area. Montana experienced a very warm month. Many locations in the West experienced damaging high winds this month associated with passing cold fronts or Santa Ana conditions.

December temperatures were below normal in much of the West with the notable exception of the upper Missouri River Basin. During the second half of the month, many locations in Montana saw positive temperature departures from normal in the double digits. Glasgow, Montana airport experienced its 11th warmest December monthly average at 23.8 F (-4.5 C) in a record beginning in 1955. A number of locations in New Mexico and southern Colorado experienced cold conditions, many 6-10 F (3-6 C) below normal. Roswell, New Mexico averaged 34.5 F (1.38 C), the 8th coolest in a record dating from 1894. With clear skies in western portions of the region, daytime highs were well above average, and nighttime lows were well below average.

Precipitation was below normal in much of the West this month. A series of storms passed through the Pacific Northwest during the last week of the month, staving off near-record dryness, but still only about half the average. A series of cutoff low-pressure systems moved across the Southwest during the first half of the month, bringing above normal precipitation to Arizona, New Mexico, and Southern Colorado. Pueblo, Colorado airport recorded its 5th wettest December with 0.84 in (21.3 mm) on a record beginning in 1954. Albuquerque, New Mexico recorded their 9th wettest December with 1.2 in (30.5 mm).

Many locations in California and Nevada experienced record or near-record dry conditions. Reno, Nevada reported its lowest December total (not even a trace) in 129 years (1883 and four prior years also had no precipitation). Fresno, California reported no measurable precipitation for the 2nd time since 1878 (tied with 1989). Downtown San Francisco received 0.14 in (3.6 mm), the 3rd driest December since 1849, behind only the 0.00 readings of 1876 and 1989. San Jose (2nd driest since 1874) and Sacramento (6th driest since 1849) also had a notably dry month, and an index of 8 Sierra Nevada stations recorded its second lowest December total since 1920, and 4th lowest July-December.

December 1-2: Santa Ana Wind Event: A strong wind event affected much of the Southwest. Gusts of 140 mph (225 kph) were recorded at the crest of the southern Sierra Nevada. Gusts to 80 mph (128 kph) were recorded in the Los Angeles area. Many power lines and trees were downed in Southern California, and on the 1st, 200,000 people were without power in the Los Angeles area. Pasadena, California declared a state of emergency following the wind event. Several wildfires broke out with the dry, warm air created near the coast. Albuquerque, New Mexico, Las Vegas, Nevada and Salt Lake City, Utah were also affected by this wind event and recorded high gusts.

December 3-5: Snowstorms in West: The low pressure system that helped to generate high winds over the Southwest moved northeast, bringing snowfall to the southern Rockies and plains. Most basins in northern Arizona and New Mexico and Colorado received 4-8 in (10-20 cm). Higher altitude locations like Taos, New Mexico received up to 18 in (45.7 cm) of snowfall.

December 11-12: Southwest Precipitation Event: Another cutoff low moved down the West Coast and then inland on December 11-12, bringing significant precipitation and snowfall to Southern California, eastern Nevada, southern Utah, and northern Arizona and New Mexico. Several inches of snow were received throughout the area, with highest storm totals of up to 8 in (20 cm) along the Arizona/New Mexico border. San Diego received a storm total of 0.67 in (17 mm), one-third of the location’s average December total of 1.78 in (45.2 mm).

December 19th: New Mexico/Southeast Colorado Snowstorm: New Mexico and Southeast Colorado received several inches of snow with totals up to 1 foot (30.4 cm) at high elevations. High winds and low visibility associated with the storm caused road closures and travel difficulties on several major travel routes.

December 30-31: Colorado/Wyoming High Wind Event: Damaging wind gusts up to 75 mph (120 kph) occurred in southern Wyoming and northern Colorado. High profile vehicles and tractor-trailers were knocked over on some Wyoming highways. A roof was torn of a warehouse in Cheyenne, Wyoming injuring two people, and a falling tree branch killed a man in Boulder, Colorado. Localized power outages were also experienced throughout the region.

PLEASE NOTE: All of the temperature and precipitation ranks and values are based on preliminary data. The ranks will change when the final data are processed, but will not be replaced on these pages. Graphics based on final data are provided on the Temperature and Precipitation Maps page and the Climate at a Glance page as they become available.

Global Analysis

Contents of this Section:

Global Highlights

The combined global land and ocean average surface temperature for December 2011 was 0.48°C (0.86°F) above the 20th century average of 12.2°C (54.0°F). This tied with 1987 and 2004 as the 10th warmest December on record.

The global land surface temperature for December 2011 was 0.88°C (1.58°F) above the 20th century average of 3.7°C (38.7°F). This tied with 1979 as the eighth warmest December on record.

The worldwide ocean surface temperature for December 2011 tied with 1972 as the 16th warmest December on record, 0.32°C (0.58°F) above the 20th century average of 15.7°C (60.4°F).

For the year (January–December), the combined global land and ocean surface temperature tied with 1997 as the 11 th warmest such period on record, at 0.51°C (0.92°F) above the 20th century average of 13.9°C (57.0°F). 2005 and 2010 are tied for warmest year on record, at 0.64°C (1.15°F) above the 20th century average.

Introduction

The data presented in this report are preliminary. Ranks and anomalies may change as more complete data are received and processed. Effective with the July 2009 State of the Climate Report, NCDC transitioned to the new version (version 3b) of the extended reconstructed sea surface temperature (ERSST) dataset. ERSST.v3b is an improved extended SST reconstruction over version 2. For more information about the differences between ERSST.v3b and ERSST.v2 and to access the most current data, please visit NCDC's Global Surface Temperature Anomalies page.

Temperature anomalies for December 2011 and January–December are shown on the dot maps in the following section. The dot maps on the left provide a spatial representation of anomalies calculated from the Global Historical Climatology Network (GHCN) version 3.1.0 dataset of land surface stations using a 1961–1990 base period. The dot maps on the right are a product of a merged land surface and sea surface temperature (SST) anomaly analysis developed by Smith et al. (2008). For the merged land surface and SST analysis, temperature anomalies with respect to the 1971–2000 average for land and ocean are analyzed separately and then merged to form the global analysis. For more information, please visit NCDC's Global Surface Temperature Anomalies page.

Temperatures

In the atmosphere, 500-millibar height pressure anomalies correlate well with temperatures at the Earth's surface. The average position of the upper-level ridges of high pressure and troughs of low pressure—depicted by positive and negative 500-millibar height anomalies on the December 2011 map—is generally reflected by areas of positive and negative temperature anomalies at the surface, respectively.

December

The December 2011 globally-averaged temperature across land and ocean surfaces was 0.48°C (0.86°F) above the 20th century average of 12.2°C (54.0°F), making this month the 10th warmest December since records began in 1880. This anomaly is significantly lower than the record December warmth in 2006, which was 0.72°C (1.30°F) above average.

Separately, the worldwide average land surface temperature anomaly was 0.88°C (1.58°F), the eighth warmest on record for the month of December. It was most notably warmer than average across much of the higher latitudes in the Northern Hemisphere, including Canada, Scandinavia, most of northern Russia, and the U.S. state of Alaska, which were more than 5°C above normal in many locations, as indicated by the dot maps above. It was also much warmer than normal across the eastern two-thirds of the United States and nearly all of Europe.

With records dating back to 1918, Alaska reported its third warmest December. The monthly statewide temperature was 4.8°C (8.7°F) above the 1971–2000 average.

In Norway, the average December temperature was 3.3°C (5.9°F) above average, making this the 20th warmest December since record keeping began in 1900. While the entire country was above average for the month, the greatest warmth was in the north of the country, where it was the 10th warmest on record for Northern Norway.

The United Kingdom reported its warmest December since 2006. This December was 0.6°C (1.1°F) above the 1971–2000 average and more than 5°C above the record cold December of 2010.

Germany was also mild, according to Deutscher Wetterdienst. The December temperature was 2.6°C (4.7°F) above the 1981–2010 national average, making this the fifth warmest December since records began in 1881.

It was colder than average in the western United States, southern Greenland, Iran, Central Asia, Mongolia, northern and southeastern China, and most of Australia.

Australia's nationally-averaged monthly temperature amounted to the coolest December since 2001, according to the Australian Bureau of Meteorology. Both Queensland and New South Wales saw their coolest December since 1999. For New South Wales, this was also the state's third coolest December on record, at 2.46°C (4.43°F) below the 1971–2000 average.

The El Niño-Southern Oscillation (ENSO) is a periodic fluctuation in sea surface temperature (El Niño) and the air pressure of the overlying atmosphere (Southern Oscillation) across the equatorial Pacific Ocean. A weak-to-moderate cold phase (La Niña) ENSO persisted through December as sea surface temperatures in the central and eastern equatorial Pacific Ocean remained cooler than normal. The December 2011 globally-averaged ocean temperature anomaly of 0.32°C (0.58°F) was the 16th warmest on record for December but was the lowest positive anomaly for any month since March 2008, also a La Niña month. The average Northern Hemisphere ocean temperature anomaly was the coolest since April 2011 while the Southern Hemisphere was the coolest since January 2008. According to NOAA's Climate Prediction Center, La Niña is expected to peak during the December–February period and continue into the Northern Hemisphere spring 2012. In addition to the equatorial Pacific, ocean temperatures were also cooler than average across the northeast Pacific, the north central and south Atlantic Ocean, and parts of the mid-latitude Southern oceans. It was notably warmer than average in the north central Pacific Ocean, the eastern Indian Ocean, and parts of the mid-latitude southern oceans. Images of sea surface temperature conditions are available for all weeks during 2011 from the weekly SST page.

Year-to-date (January-December)

The January–December map of temperature anomalies shows that warmer-than-average temperatures occurred across much of the globe during 2011. La Niña events at both the beginning and end of the year influenced global temperatures, dampening them from the record warmth observed during 2010 (which tied with 2005). Overall, the worldwide combined land and ocean surface temperature was 0.51°C (0.92°F) above the 20th century average, ranking as the 11th warmest year on record and the coolest year since 2008. This temperature anomaly is 0.13°C (0.23°F) less than the record warmth of 2010 and 2005. Separately, the 2011 average global land surface temperature ranked as the eighth warmest on record and the ocean surface ranked as the 11th warmest.

The greatest above-average annual temperature anomalies occurred across the Northern Hemisphere high latitude land areas, particularly central and northern Russia, Scandinavia, and Canada. Also noteworthy, it was warmer than average across the eastern half of the United States, Mexico, most of Europe and Africa, and the north central Pacific Ocean. Temperatures were below normal across the eastern and central Pacific Ocean, west central North America, and north and central Australia. Please refer to the NCDC State of the Climate Annual Global Analysis report for more detailed information.

Precipitation

The maps below represent anomaly values based on the GHCN version 2 dataset of land surface stations using a base period of 1961–1990. During December 2011, above-average precipitation fell over areas that included much of Australia, Alaska, the south central United States, northern Mexico, Central America, northwestern South America, and Western and Northern Europe. Drier-than-average conditions were present across the northern and far western United States, southern South America, Southern Europe, much of South and East Asia, and parts of North and West Africa.

Following the driest month ever recorded in Germany, December 2011 was much wetter than normal, at 69.1 percent above the 1981–2010 average, and ranks as the fifth wettest December in the country's 131-year period of record. Several large storms throughout the month contributed to the wetness, according to Germany's National Meteorological Service, DWD.

It was also wet in Norway. This month tied with December 1991 as the seventh wettest such period on record, at 30 percent above average, as reported by the Meteoroloisk Institutt. Several stations on the west coast recorded their wettest December on record.

Conversely, it was Spain's driest December since 1988 and fourth driest in the past half century, according to the country's national meteorological agency, AEMET. On average, Spain received 25 mm of rainfall, which is less than one-third of normal for the month.

La Niña conditions contributed to heavier-than-average rainfall over Australia in December. The country had its 25th wettest December since records began in 1900, with rainfall 34 percent above normal. With the exception of Tasmania, which experienced its driest December since 1994, all states reported above-average precipitation.

In New Zealand, precipitation varied across the country. According to NIWA, most of the North Island and northern South Island received more than two times the average monthly precipitation, while Southland and Otago on the South Island observed less than half of average rainfall.

Year-to-date (January–December)

Global precipitation over land in 2011 was well above the 1961–1990 average for the second year in a row, ranking as the second wettest year on record, behind 2010. Precipitation anomalies were variable across the globe. It was wetter than normal across much of the northeastern United States, Central America, much of coastal South America, Australia, and northwestern China. It was particularly drier than normal in the south central United States, southern and northeastern China, and Mongolia. Please refer to the NCDC State of the Climate Annual Global Analysis report for more detailed information.

Global Hazards

Please note: Material provided in this report is chosen subjectively and included at the discretion of the National Climatic Data Center (NCDC). The ability to report on a given event is limited by the amount of information available to NCDC at the time of publication. Inclusion of a particular event does not constitute a greater importance in comparison with an event that has not been incorporated into the discussion. Data included in this report are preliminary unless otherwise stated. Links to supporting information are valid at the time of publication, but they are not maintained or changed after publication.

Parts of Central and Eastern Europe were suffering from a major drought in December. In Serbia, the Danube River—one of Europe's largest rivers and busiest shipping routes—fell to record low levels, stranding about 80 cargo ships on the Serbia-Hungary border. Sunken World War II ships surfaced on the river and unexploded bombs that fell during the 1940s were found along the Sava River, also in Serbia. In Bulgaria, the Danube was at its lowest level in 70 years and shipping in many areas was forced to halt. Transport was also affected along the upper Danube in Austria and Germany. Cargo shipping in Austria was only at about 25 percent of normal volume. Shipping was also disrupted in Germany along the Rhine and Elbe rivers due to low water levels. According to a media report, the Czech Republic was experiencing its worst drought since records began in 1775. In Bosnia, drinking water restrictions were in place in Sarajevo and other cities.

In Antarctica, the South Pole recorded its all-time highest temperature—9.8°F (-12.3°C)—on December 25th, breaking the previous record of 7.5°F (-13.6°C) set on December 27th, 1978. The average temperature this time of year is about -15°F (-26°C). Summer reaches its peak during mid-December to mid-January in this region. It also snowed that day, although the South Pole is typically one of the driest places on Earth. The record "warmth" was due to incoming winds from an unusual direction, according to a senior meteorologist at the Amundsen-Scott South Pole station.

Heavy rains caused a mudslide on December 8th that killed six people about 200 miles (320 km) south of Bogota, Colombia. Officials reported that 3,500 homes were flooded, with water as high as 5 feet (1.5 meters). Including this latest tragedy, heavy rains in the country have led to at least 140 deaths since September.

A severe wind storm in the western United States caused significant damage in parts of California, Utah, Nevada, Arizona, New Mexico, Colorado, and Wyoming during November 30th–December 2nd. The strong Santa Ana winds resulted from a large difference in pressure between a strong, high pressure system and a cold, low pressure system. The high winds toppled thousands of trees, knocked power out to hundreds of thousands of residents, blew over more than a dozen semi-trailer trucks in California and Utah, and caused several areas, including Los Angeles County, to declare states of emergencies. Wind gusts surpassing 140 mph (225 km/hr)—equivalent to major hurricane-force winds—were measured in California on the Sierra Crest mountain ridge.

Tropical Storm Washi (locally referred to as Sendong) made landfall on December 16th over the southern Philippine island of Mindanao, a region unaccustomed to the tropical cyclones that frequently impact more northern regions of the Philippine archipelago. In addition to the uncommon location of the storm track, December cyclones in the Western Pacific basin are relatively uncommon, with only 15 reported from 1978 to 2010. Preliminary reports indicate that at least 1.257 residents perished due to major landslides. Thousands more were injured. Cagayan de Oro and nearby Iligan in northeastern, coastal Mindanao, both built on the mouths of major rivers, were among the hardest hit. An estimated 5.6 inches (142 mm) of rain was recorded at Lumbia Airport in Cagayan de Oro in a 12-hour period. The heavy rain caused landslides and rivers to swell and overflow their banks. Eyewitnesses reported that water rose 11 feet (3.3 meters) in less than an hour in a section of Cagayan de Oro. According to reports, the devastating death toll was due to a combination of factors, including topography (this is a region where forests have been clearcut and pineapple and banana plantations have been built on mountainsides, making the area prone to landslides and floods), the timing of the storm (Washi struck during the night while many residents were asleep), poor housing materials and locations within flood plains, and a lack of preparedness and adherence to storm warnings. Reports stated that entire villages were destroyed. Damages were estimated to be close to $23 million U.S. dollars. Prior to Washi, La Niña conditions had already brought wetter-than-normal conditions to the region. These conditions were forecast to continue through December, according to the Philippine national meteorological agency, PAGASA.

Flash Flooding Katherine, Australia27 December 2011Credit: AFP

In northern Australia, Tropical Cyclone Grant brought heavy rain and destructive winds that reached 80 mph (130 km/hr) on December 25th near the Tiwi islands north of Darwin. The storm caused flash flooding in the Northern Territory, knocking a freight train off a bridge and destroying large parts of a major highway, effectively shutting down transport routes to the region. One person was killed after their boat capsized off Whitsunday Island near the Queensland coast. The storm crossed into the Gulf of Carpenteria, weakening into a monsoonal low, and was expected to make landfall on December 30th over an uninhabited region of Cape York.

Eastern Australia was simultaneously affected by a different storm—the remnants of Tropical Storm Fina. Fina created 13 foot (4 meter) waves along the northeastern coastline in Queensland, forcing many beaches to close from December 25th to 28th due to the large swells and dangerous surf conditions. In the southeast, large hailstones shattered glass in cars and buildings in Melbourne and a tornado touched down in nearby Fiskville. No fatalities were reported.

In the Bay of Bengal, Tropical Storm Thane made landfall on December 30th over southeastern India near the coastal town of Pondicherry (Puducherry). Torrential rains and sustained winds of 46 mph (74 km/hr) damaged homes. The storm was blamed for at least 42 deaths. The storm moved westward over land and quickly dissipated.

Severe extra-tropical cyclone Joachim struck Western Europe on December 15th–17th,. Heavy rain and snow, combined with high winds, caused travel disruption and hundreds of thousands of power outages across northern and western France, Germany, and Switzerland. The storm brought wind gusts as high as 94 mph (150 km/hr)—equivalent to the strength of a Category 1 hurricane—and pressure as low as 963.8 millibars. No fatalities were reported. The storm made its way into the Austrian and Swiss Alps, bringing welcome snowfall to resorts that had started off the winter ski season with unusual dryness. Please see above for more information on the European drought.

Storm Dagmar pounded Nordic countries in northern Europe on December 25th, cutting off electricity to an estimated 264,500 residents across Norway, Finland, and Sweden. The storm was the worst to hit the region since Janika in November 2001. Wind gusts of 145 mph (233 km/hr) were recorded in Norway, with average sustained winds up to 100 mph (161 km/hr). In middle and northern Sweden, trains were halted and roads were temporarily closed. No injuries or fatalities were reported. According to the Finnish Meteorological Institute, storms of this magnitude "are rare but not exceptional."

In the United States, a major winter storm brought blizzard conditions to parts of the Southern Rockies and Central Plains regions of the country on December 19th–20th. Portions of major interstates and other roads and highways were closed from New Mexico to Kansas, stranding many holiday travelers. At least seven deaths were blamed on the storm. According to the NOAA's Hydrometerological Prediction Center, Pietown, New Mexico received 24 inches (61 cm) of snowfall, while several other areas in New Mexico and Colorado received at least 12 inches (30 cm).

A cold wave during the second half of December killed at least 135 people in northern India, primarily in the state of Uttar Pradesh. Thick fog brought transportation systems to a halt, with passengers stranded at the railway stations and airports for several hours.

National Snow & Ice

NCDC transitioned to the nClimDiv dataset on Thursday, March 13, 2014. This was coincident with the release of the February 2014 monthly monitoring report. For details on this transition, please visit our public FTP site and our U.S. Climate Divisional Database site.

During December, the weather pattern brought several strong snow storms to the Southern Rockies and parts of the High Plains. A large and persistent trough in the jet stream allowed cooler-than-average temperatures to be present across most of the West, and as storms moved onshore from the Pacific, precipitation fell mainly as snow across the Southwest. Warmer-than-average temperatures across the Northern Plains, Midwest, and most of the East caused precipitation to fall predominantly as rain when these storms systems moved eastward. This limited snow cover for the eastern two-thirds of the nation during December. At the beginning of December, according to NOAA’s National Snow Analysis, 17.2 percent of the contiguous U.S. was snow covered. Snow cover was present across the high terrain of the West, while light snow depths were observed across the Northern Plains, the Central Great Lakes, the Mid-Mississippi River Valley, and along the Central Appalachians. By December 31st, 18.9 percent of the contiguous U.S. was snow covered — the high terrain of the West, parts of the Northern Plains and Great Lakes, and parts of the Northeast and northern New England.

According to data from the Rutgers Global Snow Lab, a NOAA supported facility, snow cover extent across the contiguous U.S. during December was below average. The monthly snow cover extent was 277,300 square miles below the 1981-2010 average, and ranked as the 11th smallest December snow cover extent in the 46-year period of record. This is the first December since 2006 with below-average snow cover extent for the contiguous United States. Snow coverage was below average across the Northeast, the Great Lakes, the Central and Northern Plains, the Great Basin, the Sierra Nevada, and Cascades. Above-average snow cover was observed across parts of the Ohio Valley, the Southern and Central Rockies, and the High Plains.

Summary of Notable Snow Events:

A strong and powerful winter storm moved across the Southern Rockies and High Plains of Colorado, New Mexico, Texas, Oklahoma, and Kansas on December 19th and 20th. Snowfall up to 15 inches was reported across the region, and strong winds in excess of 50 mph created blizzard conditions. Parts of Interstates 25 and 70 were closed, and the storm was blamed for six deaths. Dalhart, Texas, shattered its daily snowfall record, where 7.0 inches fell on the 19th, surpassing the 0.1 inch which fell on the date in 1983. The 8.2 inches that accumulated in Pueblo, Colorado during the two-day event contributed to the city having its snowiest December on record, with 18.6 inches falling throughout the month. This surpassed the previous December record of 18.2 inches, which fell in December 1913.

Synoptic Discussion

Note: This Synoptic Discussion describes recent weather events and climate anomalies in relation to the phenomena that cause the weather. These phenomena include the jet stream, fronts and low pressure systems that bring precipitation, high pressure systems that bring dry weather, and the mechanisms which control these features — such as El Niño, La Niña, and other oceanic and atmospheric drivers (PNA, NAO, AO, and others). The report may contain more technical language than other components of the State of the Climate series.

NCDC transitioned to the nClimDiv dataset on Thursday, March 13, 2014. This was coincident with the release of the February 2014 monthly monitoring report. For details on this transition, please visit our public FTP site and our U.S. Climate Divisional Database site.

Synoptic Discussion

Upper-level circulation pattern and anomalies averaged for December 2011.

Like last month, a vigorous weather pattern dominated the contiguous United States during December 2011. Several strong upper-level low pressure systems dug into the Southwest then trekked toward the Great Lakes, dropping a wintry blanket of snow that covered nearly 39 percent of the country by December 6 and 31 percent on December 24. Cold air behind the upper-level lows brought below-normal December temperatures to the West and Southern Plains, while warm air advection ahead of them resulted in above-normal temperatures across the East and Midwest to the Northern Plains. This storm track kept moisture away from the West Coast, Interior West, Northern Plains, and Coastal Southeast, where December precipitation totals were below normal, and resulted in several tornadoes in the Southeast later in the month.

Cold fronts and low pressure systems moving in the storm track flow are influenced by the broadscale atmospheric circulation.
Four such large-scale atmospheric circulation drivers were potentially influential during December:

Ocean temperatures and atmospheric circulation anomalies indicated that the equatorial Pacific continued in a La Niña state. La Niña this time of year (October-December) is associated with temperature and precipitation anomalies across the U.S. — temperatures are typically warmer than normal from the Mississippi River to the Interior West, with drier than normal conditions dominating most of the country, and wet conditions limited to the Pacific Northwest and northern California.

The December 2011 averaged upper-level circulation anomalies are consistent with a positive AO pattern across the northern and eastern United States. The pattern of observed temperature anomalies for December 2011 and the last three months (October-December) corresponds to the La Niña and AO patterns in the Northern Plains to Great Lakes and NAO from the Northern Plains to the East Coast. The December 2011 and October-December 2011 precipitation patterns are a reasonable match for the La Niña pattern across the mid-Gulf to Southeast coasts, the PNA and NAO patterns for part of the West, and the AO pattern for the mid-Gulf Coast and Upper Mississippi Valley. However, the unusually wet December across the Southwest and Southern to Central Plains does not correlate with any of these four atmospheric patterns, but instead is more reminiscent of a springtime El Niño influence. The Ohio Valley had above-normal precipitation during December and October-December 2011. Below-normal precipitation, not above-normal precipitation, in the Ohio Valley is associated with La Niña and a positive AO, NAO, and PNA in October-December. Wet conditions here are more likely in the late winter to early spring with La Niña and positive AO, and during the spring with a neutral AO.

Tornadoes

NCDC transitioned to the nClimDiv dataset on Thursday, March 13, 2014. This was coincident with the release of the February 2014 monthly monitoring report. For details on this transition, please visit our public FTP site and our U.S. Climate Divisional Database site.

According to data from the Storm Prediction Center, the preliminary count of tornado reports during December 2011 — 33 — was slightly above the 1991-2010 average of 24. Most of the month’s tornadoes occurred during a single outbreak on December 22nd, when a cold front pushing through the Southeast spawned 12 preliminary reports of weak tornadoes from Louisiana to Georgia. The preliminary tornado reports during December pushed the annual tornado count upwards slightly. During the January-December period, 1,625 tornadoes have been confirmed across the country, with 93 reports still pending for November and December. The will rank 2011 as the second or third most active tornado year on record, depending on how many tornadoes are confirmed from the last two months of 2011. There were 1,817 tornadoes confirmed in 2004 and 1,689 tornadoes in 2008. The modern torando record dates back to 1950.

Hurricanes & Tropical Storms

NCDC transitioned to the nClimDiv dataset on Thursday, March 13, 2014. This was coincident with the release of the February 2014 monthly monitoring report. For details on this transition, please visit our public FTP site and our U.S. Climate Divisional Database site.

Note:This report catalogs recent tropical cyclones and places each basin's tropical cyclone activity in a climate-scale context. It is not updated in real time. Users seeking real time status and forecasts of tropical cyclones should visit The National Hurricane Center.

Drought

NCDC transitioned to the nClimDiv dataset on Thursday, March 13, 2014. This was coincident with the release of the February 2014 monthly monitoring report. For details on this transition, please visit our public FTP site and our U.S. Climate Divisional Database site.

Historical temperature, precipitation, and Palmer drought data from 1895 to present for climate divisions, states, and regions in the contiguous U.S. are available at the Climate Division: Temperature-Precipitation-Drought Data page. These filenames begin with "drd964x" and end with "txt".

December 2011 was warmer and drier than average (34th warmest and 46th driest, based on data back to 1895) when weather conditions are averaged across the country. But this reflected persistent weekly regional patterns of temperature anomalies (weeks 1, 2, 3, 4, 5) and precipitation anomalies (weeks 1, 2, 3, 4, 5). Much of the West was drier than normal for the first four weeks, with weather systems bringing above-normal weekly precipitation to the northern areas during the last week of the month. Likewise, the first four weeks were persistently wetter than normal for much of the Southwest and Southern Plains drought areas and the last four weeks drier than normal for the Southeast coast. Above-normal temperatures dominated the Northern Plains and much of the East for most of the month, while normal to colder-than-normal temperatures were the rule for the southwestern third of the nation. These weekly patterns are reflected in the monthly patterns of temperature and precipitation. As a result, drought areas contracted in the Southwest and Southern Plains where beneficial precipitation fell. But persistent dryness in the Far West resulted in expansion of moderate drought and abnormally dry areas, with Oregon having the eighth driest start to the water year (October-December) in the 1895-2011 record and California ranking eleventh driest. Even with a wet December, long-term deficits still left the Southern Plains exceptionally dry. Rainfall over parts of Hawaii shrank the moderate to extreme drought area from 54 percent last month to 47 percent this month. Drought areas held steady or slightly increased along the Southeast coast and in the Upper Mississippi Valley to Northern Great Plains where December was drier than normal. Nationally, the moderate-to-exceptional drought footprint increased to about 27 percent of the country but the percentage in the worst category (D4, exceptional drought) dropped to about 3 percent.

Palmer Drought Index

The Palmer drought indices measure the balance between moisture demand (evapotranspiration driven by temperature) and moisture supply (precipitation). The Palmer Z Index depicts moisture conditions for the current month, while the Palmer Hydrological Drought Index (PHDI) and Palmer Drought Severity Index (PDSI) depict the current month's cumulative moisture conditions integrated over the last several months.

As seen on the December 2011 Palmer Z Index map, low precipitation led to short-term drought across the Upper Mississippi Valley, western Great Lakes, Southeast Coast, Central Gulf Coast, and much of the West this month. Wet conditions are evident on the Z Index map across the Southwest and Southern Plains to Ohio Valley. Compared with the November 2011 PHDI map, the December 2011 PHDI map indicates that drought conditions intensified along parts of the Southeast Coast, Central Gulf Coast, and Upper Mississippi Valley but decreased in intensity in parts of the Southwest and Southern to Central Plains, and wet conditions decreased in parts of the West with moderate drought developing in the Far West.
The December 2011 PHDI map also reflects the long-term nature of the drought conditions. The Z Index and PHDI maps in combination show that precipitation brought relief to parts of the Southwest to Central and Southern Plains drought area, and moisture conditions further declined across much of the Far West; but for the Upper Mississippi Valley, Southeast Coast, Central Gulf Coast, and Ohio Valley — precipitation fell where it was already wet and it was drier than normal over the existing drought areas.

Standardized Precipitation Index

The Standardized Precipitation Index (SPI) measures moisture supply. The SPI maps here show the spatial extent of anomalously wet and dry areas at time scales ranging from 1 month to 24 months.

Dryness is evident across much of the West at the 1 to 6 month time scales, and parts of the West at 9 months and even as far back as 12 months; Northern Plains dryness is evident at 1 to 3 months, with part of the Northern Plains into the adjoining Midwest dry at 6 months; dryness can be seen in the Upper Mississippi Valley and adjoining western Great Lakes at 1 to 12 months; northern New England is dry at the 2 month time scale; and the Southeast Coast and Central Gulf Coast dryness shows up at 1 to 3 months with the dry area expanding across much of the Southeast at 6 to 24 months. For the Southwest and Southern to Central Plains, beneficial December precipitation is reflected in wet to neutral conditions on the December (1 month) map back as far as 3 months, but the prolonged deficits make themselves evident as dryness at 6 to 24 months. Wet conditions caused by several frontal and low pressure systems can be seen along bands from the Southwest and Southern Plains to Ohio Valley at 1 to 2 months. Wet conditions caused by weather systems earlier in the year are evident in the Ohio Valley to Northeast at 3 to 24 months and in the Northern Plains at 9 to 12 months. This illustrates the persistence of the dry and wet areas. But the wet conditions from last winter's above-normal precipitation don't show up in the West until the 24 month map, illustrating the magnitude and impact of the early season dryness during this winter.

Agricultural and Hydrological Indices and Impacts

NLDAS soil moisture percentiles

USGS monthly streamflow percentiles

Drought conditions were reflected in numerous agricultural, hydrological, and other meteorological indicators, both observed and modeled.
Across the drought areas of the Southeast, streamflows were low and many groundwater well stations were at or near record low levels for this time of year. Low streamflows also characterized the Southern Plains drought area and the developing drought areas of the West Coast. Soils were still dry in the Southern Plains (in spite of recent precipitation) to the Southeast, and soils continued drying out in the Upper Mississippi Valley and now in the Far West.
Parts of the West, Northern Great Plains, Upper Mississippi Valley, and coastal Southeast had few, if any, days with precipitation in December.
This summary is based on the following observed and modeled indicators:

Most stations in Alaska were wetter than normal this month, resulting in the fifth wettest December in the 1918-2011 record, statewide. In spite of the recent wetness, precipitation deficits are evident at interior stations, especially at longer time scales (2, 3, 6, 12, 24, 36 months). However, snowpack and snow water content (for stations and basinwide) were generally near to above normal, and there was no drought or abnormal dryness indicated on the January 3rd USDM. Modeled soil moisture was still drier than normal in south central coastal to interior locations.

The southeastern and northwestern thirds of Puerto Rico were drier than normal during December. The dryness is evident at 2 and 3 months, especially in the southeast, but disappears at longer time scales (6 and 12 months). With streamflow near average, the January 3rd USDM map had no drought or abnormally dry areas on the island.

On a statewide basis, December 2011 ranked in the top ten driest Decembers for eight states in the West and Northern Plains, with two other states close behind (Minnesota at 12th driest and North Dakota at 13th driest). Five other states (including three in the Southeast — North and South Carolina and Georgia) ranked in the driest third of the historical record.
The dryness in the West and Northern Plains to Upper Mississippi Valley extends back five to six months. Minnesota had the sixth driest start (October-December 2011) to the water year (October through the following September), with Oregon ranking eighth driest and California 11th driest. Three states in the West (Oregon at third driest, Idaho at fifth driest, and California at seventh driest) and one in the Upper Mississippi Valley (Minnesota at ninth driest) ranked in the top ten driest for the last six months (July-December 2011), with Montana and Georgia close behind at eleventh driest.

Beneficial December precipitation in the Southwest and Southern Plains improved the rankings for New Mexico and Texas. Statewide precipitation ranks for Texas are in the top ten driest category for July-December 2011 back to January-December 2011, but not for the time periods from August-December 2011 to December 2011. For New Mexico, only March-December, February-December, and January-December have 2011 statewide precipitation ranks in the top ten driest category. But the long-term deficits in Texas were so severe that, even with the December precipitation, the Lone Star State still had the driest March-December, February-December, and January-December in the 1895-2011 record.
Four other states ranked in the top ten driest category for the year, including Georgia (fifth driest), New Mexico (sixth driest), Louisiana (seventh driest), and South Carolina (eighth driest), with Oklahoma close behind at eleventh driest.
The December 2011 precipitation was enough to raise the annual (January-December 2011) ranking for the Rio Grande and Texas Gulf Coastriver basins to second driest year in the 1895-2011 record.

As noted by the Southeast Regional Climate Center, for the third straight month, precipitation was highly variable across the Southeast region. Monthly totals were between 100 and 150 percent of normal across western portions of Virginia and North Carolina and across sections of Alabama and North Georgia. Some locations across the Southern Appalachians recorded between 150 and 300 percent of normal precipitation for the month. In contrast, the driest locations across the Southeast were found across the eastern Carolinas and the Florida Peninsula, where monthly precipitation totals were less than 25 percent of normal. Charleston, South Carolina recorded only 0.65 inch (16.5 mm) of precipitation, making it the second driest December in a record extending back to 1938. Elsewhere across the region, monthly precipitation was between 25 and 75 percent of normal, including the western slopes and interior mountains of Puerto Rico.

Monthly average temperatures for December were above normal across the Southeast region. December ranked among the top 10 warmest on record at several locations across the region. Overnight temperatures were especially warm during the month, as over 200 daily high minimum temperature records were tied or broken across the region. Many of these occurred in the days before Christmas, which was in stark contrast to the cold and snow that struck the region one year earlier. The calendar year 2011 was exceptionally warm across a large portion of the Southeast region.

Drought conditions persisted across more than half of the Southeast region in December. The lack of rainfall across the Florida Peninsula contributed to the re-emergence of D0 (abnormally dry) conditions by the end of the month, forcing several communities to consider implementing additional water restrictions. The warm, dry weather facilitated the planting and growth of winter grains and vegetables, as well as the transport of harvested crops across the region.

As explained by the Southern Regional Climate Center, with a few exceptions, December was generally a wet month for the Southern region. Dry areas included southern Louisiana and southern Mississippi, eastern Oklahoma, and parts of western Texas. These areas generally averaged approximately 5-25 percent of normal precipitation for the month. The wettest part of the region included Arkansas, parts of southern and northern Texas, and western Oklahoma. Stations in these regions averaged between 150 and 400 percent of normal precipitation. Texas averaged 2.88 inches (73.15 mm) of precipitation, making it the nineteenth wettest December on record (1895-2011). It is also the second month in the year where precipitation for the state exceeded two inches (50.80 mm). Oklahoma experienced its 28th wettest December on record (1895-2011) with a state average precipitation total of 2.35 inches (59.69 mm). For Arkansas, it was the eighth wettest December on record (1895-2011) with a state average precipitation total of 7.47 inches (189.74 mm). Stations in the northeastern and southern areas of the state averaged between 150 and 250 percent of normal. Louisiana averaged 5.49 inches (139.45 mm) of precipitation, while Mississippi, averaged 5.99 inches (152.15 mm) of precipitation. Tennessee averaged 5.91 inches (150.11 mm) of precipitation for the month. State rankings for Louisiana, Mississippi, and Tennessee were all in the middle two quartiles. December was generally cooler than normal in Texas, northern Louisiana and in the Oklahoma panhandle. Elsewhere, conditions were generally slightly warmer than normal.

Drought conditions in the Southern region improved slightly from the previous month. Above-normal precipitation over much of the region led to improvements over western Louisiana, northwestern Texas, and northern Texas. As of January 3, 2012, only 17.24 percent of the Southern region was experiencing exceptional drought, which is approximately a twelve percent improvement over the end of November. The amount of extreme drought was also reduced by approximately eleven percent. Drier-than-normal conditions in southeastern Louisiana, however, resulted in the introduction of extreme drought over much of the Florida Parishes.

As summarized by the Midwest Regional Climate Center, December precipitation ranged from less than 25 percent of normal in some parts of Minnesota to more than twice normal in northwest Missouri and southern Iowa. Annual precipitation ranged from less than 75 percent of normal in northwest Minnesota to 150 percent of normal along the Ohio River Valley. Indiana, Ohio, and Kentucky recorded their wettest year statewide in 2011 (1895-2011 period). Ten cities along the Ohio River and northward to Lake Erie set new records for annual precipitation. December temperatures averaged 3 to 10 degrees F (2 to 6 degrees C) above normal across the Midwest with the largest departures in Minnesota. Temperatures were below normal early in December but well above normal for the last two-thirds of the month. Daily temperature records were almost exclusively record highs with 629 record highs and just 7 record lows. Annual temperatures were very close to normal for the entire region with all locations within 2 degrees F (1 degree C) of normal for the year.

As noted by the Northeast Regional Climate Center, December's precipitation total of 3.64 inches (92.5 mm) was 104 percent of normal. While this month's total was 0.17 inch (4.3 mm) above normal, it was 0.32 inch (8.1 mm) less than December 2010. This was the fifth consecutive wetter-than-normal December for the region. None of the states' precipitation totals departed far from normal — Delaware, at 94 percent of normal, was the driest state; and Pennsylvania was the wettest, with 113 percent of its normal precipitation amount. The Northeast's annual precipitation total of 55.14 inches (1400.6 mm) sent 2011 into the record book as the wettest year since 1895, surpassing 1996's total of 54.60 inches (1386.8 mm). This year's total was 10.97 inches (278.6 mm) wetter than normal. Four of the states hit hard by the tropical storms in August and September — Connecticut, New Jersey, New York, and Pennsylvania — also had their wettest year on record. Vermont saw its second wettest year since 1895 and Massachusetts its third. The region averaged 124 percent of normal in 2011; state percentages ranged from 109 percent in Delaware to 136 percent in New Jersey and Pennsylvania.

December's average temperature of 34.1 degrees F (1.2 degrees C) was 5.3 degrees F (2.9 degrees C) above normal and 9.0 degrees F (5.0 degrees C) warmer than December 2010. It was the warmest December since 2006 (2006 was the warmest December since 1895) and the sixth warmest December since recordkeeping began in 1895. In addition, it was the ninth consecutive month to average warmer than normal. Each of the states in the Northeast was warmer than normal. The Northeast's average annual temperature in 2011 was 49.0 degrees F (9.4 degrees C), which was 1.5 degrees F (0.8 degrees C) above normal and only one degree F (0.6 degrees C) less than the warmest year, 1998. This year was the Northeast's 9th warmest since 1895.

As explained by the High Plains Regional Climate Center, December was an interesting month across the High Plains Region. The northern areas of the region were generally drier and warmer than normal and many areas did not experience a White Christmas. The lack of snow cover, not just in the High Plains Region, but also in areas north into Canada, contributed to the unseasonably warm temperatures. The Dakotas had the largest temperature departures in the region, ranging from near normal in the southwest corner of South Dakota to over 12 degrees F (6.7 degrees C) above normal in northern and northeastern North Dakota. Meanwhile, the western areas of the region were generally colder than normal with temperature departures at least 6 degrees F (3.3 degrees C) below normal occurring in areas of southern Colorado and Wyoming.

Precipitation was varied across the region this month. The Dakotas were generally drier than normal, with a few isolated pockets receiving above normal precipitation. The ongoing dry conditions in eastern North Dakota led to the expansion of moderate drought conditions (D1). Meanwhile, heavy rain and snow across Kansas and southern Colorado helped alleviate and in some cases eliminate drought conditions. Precipitation was a welcome sight in drought-stricken areas of Colorado, Kansas, and southeast Nebraska. This month, the heaviest precipitation in the region fell in a large swath that stretched from southeastern Colorado, through Kansas, and into southeastern Nebraska. The majority of the locations in this swath received 200-400 percent of normal precipitation, while clusters of locations in Kansas and Colorado received 400-800 percent of normal precipitation. Numerous locations ranked in the top 10 wettest Decembers on record and many had precipitation totals which were ranked second and third wettest. Snowfall totals for the month of December also ranked in the top 10 across some areas of Kansas and Colorado. At least one location had its snowiest December on record — Pueblo, Colorado. Pueblo received 18.6 inches (47 cm) of snow this month, which beat out the old record of 18.2 inches (46 cm) set all the way back 1913 (period of record 1888-2011).

The USDM had many changes this month. Storm systems bringing beneficial rain and snow to Colorado and Kansas led to the erasure of all exceptional drought conditions (D4) in those states. In addition, extreme drought conditions (D3) were also erased in Colorado. Other major changes occurred in Kansas as well, as all drought conditions were eliminated in the north and the drought conditions that remained were contracted to the south somewhat. Southeastern Nebraska also benefitted from the December precipitation as drought conditions were erased for a large portion of that area. Meanwhile, due to an ongoing lack of precipitation in North Dakota, moderate drought conditions (D1) expanded northward in the eastern side of the state. Drought conditions in South Dakota remained largely the same since last month, with the exception of a slight expansion of abnormally dry (D0) and D1 conditions in the southeast.

As summarized by the Western Regional Climate Center, dry and generally cool conditions were observed in much of the West this month as high pressure dominated the region. A series of cutoff low-pressure systems moved across the Southwest during the first half of the month, bringing above normal precipitation to Arizona, New Mexico, and Southern Colorado. But precipitation was below normal in much of the rest of the West this month. A series of storms passed through the Pacific Northwest during the last week of the month, staving off near-record dryness, but still the region had only about half the average for the month. Many locations in California and Nevada experienced record or near-record dry conditions. Reno, Nevada reported its lowest December total (not even a trace) in 129 years (1883 and four prior years also had no precipitation). Fresno, California reported no measurable precipitation for the second time since 1878 (tied with 1989). Downtown San Francisco received 0.14 in (3.6 mm), the third driest December since 1849, behind only the 0.00 readings of 1876 and 1989. San Jose (second driest since 1874) and Sacramento (sixth driest since 1849) also had a notably dry month, and an index of 8 Sierra Nevada stations recorded its second lowest December total since 1920, and fourth lowest July-December.

December temperatures were below normal in much of the West with the notable exception of the upper Missouri River Basin. During the second half of the month, many locations in Montana saw positive temperature departures from normal in the double digits. A number of locations in New Mexico and southern Colorado experienced cold conditions, many 6-10 F (3-6 C) below normal. With clear skies in western portions of the region, daytime highs were well above average, and nighttime lows were well below average.

Upper Colorado River Basin: As reported by the Colorado Climate Center, the January 3rd NIDIS (National Integrated Drought Information System) assessment for the Upper Colorado River Basin (UCRB) indicated that, for the month of December, precipitation favored the southern and western portions of the UCRB. The San Juan mountains and the Four Corners region received around 100 percent of its average December precipitation. Areas in eastern Utah and southwest Wyoming received over 150 percent of average precipitation for the month. Northwest Colorado was much drier in December, with most areas receiving less than 50 percent of average. The drought-stricken southeast Colorado saw significant improvement, with most of the region receiving over 200 percent of average precipitation for the month. Water-year-to-date SNOTEL precipitation was near average for the southern part of the UCRB and below average for much of the northern portions of the basin. Around the headwaters of the Colorado River, very little snow has accumulated since the beginning of December. With accumulations stagnating, the headwaters region (along with many other central and northern regions of the UCRB) was experiencing below average snowpack. Snowpack accumulations were now less than they were at the end of December during the 2002 drought year. As of January 1st, 91 percent of the USGS streamgages in the UCRB recorded normal (25th-75th percentile) or above normal 7-day average streamflows, while 9 percent of the gages in the basin were recording below normal flows. Much of the UCRB experienced colder-than-average temperatures for the month of December. The VIC model continued to show dry soil moisture conditions in southeast Colorado and in Utah around the Colorado River valley. Drying conditions were also showing up in southern Wyoming. Even though wet soils were still being observed over the Colorado Headwaters region, the spatial extent was decreasing and the soils were beginning to dry a little. All of the major reservoirs above Lake Powell were above their January averages. Except for Navajo and Lake Granby, all of the major reservoirs in the UCRB were above their storage levels for the same time last year. Flaming Gorge, Granby, Navajo and Dillon have stayed near steady for the month, while Blue Mesa, Green Mountain, and Lake Powell have seen larger decreases. Lake Powell was currently at 66 percent of capacity and 86 percent of average.

Pacific Islands: According to reports from National Weather Service offices, the Pacific ENSO Applications Climate Center (PEAC), and partners, conditions varied across the Pacific Islands.

As noted by the National Weather Service office in Honolulu, persistent trade winds brought abundant amounts of rain to the east-facing slopes of the main Hawaiian Islands. Many of the leeward slopes, especially on the Big Island and Maui, received very little rain. As a result, extreme drought, or D3 category conditions on the USDM map, continued in the south Kohala district and the Pohakuloa region of the Hamakua district. Some of the windward rainfall managed to push into leeward areas of the north Kohala district resulting in a retreat of drought conditions from this portion of the Big Island. Rainfall also occurred over the Kau district in sufficient amounts to result in an improvement from extreme drought to severe drought, or D2 category conditions. Severe drought continued in the north Kona district while moderate drought, or D1 category conditions, remained in place over portions of the south Kona district.
Like the Big Island, west-facing slopes of Maui stayed dry in December despite wet conditions along the east-facing slopes. Extreme drought maintained its grip on a small section of southwestern Haleakala from Kamaole to Ulupalakua and severe drought continued over the lower elevations of west Maui from Maalaea through Lahaina.
The increase in windward rainfall helped raise water levels in the Waimanalo reservoir but not enough to remove the severe drought classification for east Oahu.

Some drought impacts in Hawaii include the following:

On Kauai, there were no significant drought impacts as conditions have improved over the past month.

On Oahu, the water supply in the Waimanalo reservoir increased enough during December to warrant an easing of mandatory water use restrictions from a 20 percent cutback to a 10 percent cutback.

On Molokai, no significant changes since the December 8 update. The State of Hawaii Department of Agriculture continued to keep in place the mandatory 30 percent cutback in irrigation water consumption for the Kualapuu reservoir system.

On Lanai, no significant changes since the December 8 update. No additional impact information has been received.

On Maui, pastures and vegetation remained dry over the leeward slopes of Haleakala especially from Kamaole to Ulupalakua. Water supply levels for upcountry Maui improved significantly during December due to an increase in windward Haleakala rainfall. However, the Maui County Department of Water Supply has maintained their call for a 5 percent reduction in water use. The request for a 10 percent reduction in water use by central and south Maui residents also remained in effect.

On the Big Island, pastures and general vegetation from Kawaihae to Pohakuloa were in very poor condition and brush fires continued to be a significant concern. Ranchers in these areas have already destocked cattle and water hauling operations have been ongoing for several months. South Kau finally received significant rainfall and some recovery has occurred but more rainfall is needed.

On other Pacific Islands, December was drier than normal in Pago Pago and Kwajalein and slightly below normal in Koror, but near to above normal at most other locations. Annual 2011 rainfall was near to above normal at most locations.

Global Snow & Ice

NH Snow Cover Extent

The Northern Hemisphere snow cover extent during December 2011 was below average, marking the 13th smallest December snow cover extent in the 46-year record. This is the first December since 2006 with below average snow cover for the Northern Hemisphere. During the month, the Northern Hemisphere snow cover extent was 42.7 million square km (16.5 million square miles) — 546,000 square km (210,800 square miles) below average.

During December, the North American snow cover extent was below average, ranking as the 11th smallest December snow cover extent on record. This was the first December with below-average snow cover for North America since 2006. The monthly snow cover extent was 16.1 million square km (6.2 million square miles) — 569,000 square km (216,700 square miles) below average. During the month, several storms impacted the Southern Rockies of the U.S., causing above-average snow cover there. The rest of the U.S. generally had below-average snow coverage during the month. Canada had slightly above-average snow cover, with above average snow across the Canadian Rockies.

Eurasian snow cover was near-average in December, 23,000 square km (8,900 square miles) above the average of 26.6 million square km (10.3 million square miles), ranking as the 21st smallest (26th largest) monthly snow extent for the continent. This is the smallest December snow cover extent for Eurasia since 2008. Below-average snow cover was reported for most of Europe, with the exception of the Balkans. Below-average snow cover was reported for the Tibetan Plateau, the Himalayas, and western Asia. While above-average snow cover was observed across Turkey, Kazakhstan, Mongolia, and parts of central China.

Sea Ice Extent

According to the National Snow and Ice Data Center (NSIDC), the Northern Hemisphere sea ice extent — which is measured from passive microwave instruments onboard NOAA satellites — averaged for December 2011 was 12.38 million square kilometers (4.78 million square miles). This was 7.35 percent below the 1979-2000 average of 13.35 million square km (5.2 million square miles) and ranked as the fourth smallest December Arctic sea ice extent on record. December 2011 is the 17th consecutive December and 127th consecutive month with below-average Arctic sea ice extent. December Arctic sea ice extent has decreased at an average rate of 3.4 percent per decade.

Ice coverage during December was low across the Atlantic side of the Arctic in the Barents and Kara seas. The eastern portions of the Hudson Bay were not frozen over until the end of December; the bay is typically ice covered by the beginning of the month. Slightly-above-average ice extent was observed on the Pacific side of the Arctic, particularly in in Bering Sea.

The December 2011 Southern Hemisphere sea ice extent was 9.77 percent above average — the 5th largest (29th smallest) December extent on record. This is the fifth consecutive December with above-average ice extent across the Antarctic. According to the NSIDC, unusually high sea ice was observed through much of December, in particular in the northern Ross Sea and the eastern Weddell Sea. Southern Hemisphere December sea ice has increased at an average rate of 2.0 percent per decade, with significant inter-annual variability.

For further information on the Northern and Southern Hemisphere snow and ice conditions, please visit the NSIDC News page.

Upper Air

Note: Beginning in December 2010, all data are reported here with respect to the 1981–2010 base period. Prior to December 2010, radiosonde data were reported with respect to the 1961–1990 base period and satellite data were reported with respect to the 1979–1998 base period. Remote Sensing Systems continues to provide data to NCDC with respect to the 1979–1998 base period; however, NCDC readjusts the data to the 1981–2010 base period so that the satellite measurements are comparable. This change provides a more consistent comparison between the various datasets.

Note: Effective with the January 2011 report, Remote Sensing Systems (RSS) transitioned to a new version (3.3) of the RSS MSU/AMSU atmospheric temperature datasets. Information about the differences between version 3.2 and 3.3 is available.

Background Information

Temperatures above the Earth's surface are measured within the lower troposphere, middle troposphere, and stratosphere using in-situ balloon-borne instruments (radiosondes) and polar-orbiting satellites (NOAA's TIROS-N). The radiosonde and satellite records have been adjusted to remove time-dependent biases (artificialities caused by changes in radiosonde instruments and measurement practices as well as changes in satellite instruments and orbital features through time). Global averages from radiosonde data are available from 1958 to present, while satellite measurements date back to 1979.

The mid-troposphere temperatures are centered in the in the atmospheric layer approximately 3–10 km [2–6 miles] above the Earth's surface, which also includes a portion of the lower stratosphere. (The Microwave Sounding Unit [MSU] channel used to measure mid-tropospheric temperatures receives about 25 percent of its signal above 10 km [6 miles].) Because the stratosphere has cooled due to increasing greenhouse gases in the troposphere and losses of ozone in the stratosphere, the stratospheric contribution to the tropospheric average, as measured from satellites, creates an artificial component of cooling to the mid-troposphere temperatures. The University of Washington (UW) versions of the UAH and RSS analyses attempt to remove the stratospheric influence from the mid-troposphere measurements, and as a result the UW versions tend to have a larger warming trend than either the UAH or RSS versions. For additional information, please see NCDC's Microwave Sounding Unit page.

The radiosonde data used in this global analysis were developed using the Lanzante, Klein, Seidel (2003) ("LKS") bias-adjusted dataset and the First Difference Method (Free et al. 2004) (RATPAC). Additional details are available. Satellite data have been adjusted by the Global Hydrology and Climate Center at the University of Alabama in Huntsville (UAH). An independent analysis is also performed by Remote Sensing Systems (RSS) and a third analysis has been performed by Dr. Qiang Fu of the University of Washington (UW) (Fu et al. 2004)** to remove the influence of the stratosphere on the mid-troposphere value. Global averages from radiosonde data are available from 1958 to present, while satellite measurements began in 1979.

Wildfires

Overview

During December 2011, approximately 207,151 acres (83,830 hectares) burned across the country, the fifth most for the month in the 12-year period of record. The record for December acreage burned occurred in 2007, 427,160 acres (172,865 hectares). During the January-December period, 8.7 million acres (3.5 million hectares) burned across the U.S. — the 3rd most in the 12-year period of record. The most acres burned during the January-December period occurred in 2006, when 9.9 million acres (4.0 million hectares) burned nationwide.

Discussion

During December, wetter-than-average conditions were present from the Southern Rockies, stretching across the Southern and Central Plains and into the Great Lakes region. Most of the West and parts of the Southeast had below-average precipitation during December 2011. The weather pattern during December brought much warmer-than-average temperatures across the Northern Plains, stretching into the Northeast, Midwest, and Southeast. Cooler-than-average temperatures were present for much of the West and Southern Plains. See the U.S. Temperature and Precipitation discussion for more information. According to the U.S. Drought Monitor, the monthly pattern improved drought conditions across much of the Southern Plains and Southwest and worsened for the West. The percent area of the contiguous U.S. experiencing D0-D4 drought conditions grew from 41.0 percent at the end of November to 49.6 percent at the end of December. Drought conditions generally improved one to two categories during the month across Arizona, New Mexico, Oklahoma, Kansas, and Louisiana. Drought conditions developed across a large area of California, Nevada, and Oregon, where December and the two much prior were much drier than average. Drought conditions remained generally unchanged across the Upper Midwest and the Southeast.

At the beginning of December, there were three large wildfires burning across the country. Two large wildfires were active in central California, where below-average precipitation contributed to elevated fire danger and Keetch-Byram Drought Index (KBDI) values across the state. Very-low 10-hour fuel moistures and moderately-low 100-hour and 1,000-hour fuel moistures were also observed in the region on December 1st. One large fire was active in Minnesota, where drier-than-normal conditions have been present for the past several months, causing low 10-hour fuel moistures across parts of the state.